Scientists have uncovered a stunning collection of stone artifacts at least 15,500 years old from a central Texas archaeological site, proving that people were living in North America thousands of years earlier than previously thought.

The discoveries, reported this week in the journal Science, provide dramatic evidence that people had colonized the middle of the continent long before glaciers melted back to create an ice-free migration corridor from Alaska through Canada.

The inescapable conclusion essentially rewrites the first chapter of Alaska's prehistory and what's known about the initial colonization of the New World.

Since ice sheets blocked the way, the people could not have crossed the Bering Land Bridge and then trudged south as many have long argued and school children have always been taught. Instead, these Stone Age pioneers must have traveled along the Alaskan shoreline before continuing down the West Coast and spreading out to populate two continents, according to the study's lead author, Michael Waters, director of Texas A&M's Center for the Study of First Americans.

"While there is no empirical archaeological evidence that shows that people migrated along the coast, logic tells us they must have," Waters told Alaska Dispatch in an email earlier this week. "There is no other way to get people into central Texas — the corridor was closed."

As a result, the very first Americans almost certainly spent time as Alaskan beachcombers — foraging through tide pools and snatching salmon from coastal creeks along the southern rim of Beringia.

Waters and a team that included researchers from Baylor University, the University of Illinois-Chicago, the University of Minnesota, and Texas State University, began three years of excavations in 2006 at the Debra L. Friedkin site about 40 miles northwest of Austin on a terrace next to spring-fed Buttermilk Creek.

What they found was remarkable — 15,528 stone fragments and tools in an eight-inch-thick layer that hadn't been disturbed for millennia.

More than 60 "luminescence dates" — a technique that analyzes sediment and dates when it was last exposed to sunlight — showed that people dropped the artifacts on the surface of the ground more than 150 centuries ago, making the material theoldest archaeological evidence for human occupation in North America.

"It opens up and creates all sorts of new possibilities and new thinking about the first people to enter the continent," Waters told a Science-sponsored teleconference, held Wednesday morning.

Most significantly, the age of the artifacts predates the bison and mammoth hunters of the Clovis culture – long thought to be the original settlers of North America — by more than 2,500 years, Waters pointed out.

"What is special about the Debra L. Friedkin site is that it has the largest number of artifacts dating to the pre-Clovis time period, that these artifacts show an array of different technologies, and that these artifacts date to a very early time," Waters said.

Most of the artifacts were white flakes and tiny chips left on the ground after making new tools or sharpening old ones. But there were also 56 whole tools — including a spear points, knives and choppers, plus small items used for scraping, boring and cutting, Waters said.

"In general, the Buttermilk Creek Complex tools and cores are small in size and lightweight, a tool kit designed for high residential mobility," the authors wrote. "Although no organic artifacts were preserved, the Buttermilk Creek Complex stone tools have wear that is indicative of use on both soft and hard materials … suggesting that organic materials were also part of this assemblage."

"This is a mobile tool kit, something which is easily transported," Waters said Wednesday. "It is lightweight. Also there is evidence to suggest these people were making bone tools as well."

In a sense, the tool collection offered tantalizing hints about a full prehistoric life, where roaming hunters took time out to retrofit weapons, chip out new points and tweak their equipment in a creek bottom greenbelt. The Buttermilk Creek area may have been a productive spot for them — close to rock formations for new spear points, year-around water in the creek, and access to both the prairie and the Texas coastal plain.

"These people were hunters and gatherers, but they probably made rounds, so they probably were going to other places," Waters told the teleconference. But "it seemed that the Buttermilk Creek area was a place that people came back to continually. It is as if when that site was discovered or the valley was found that people kept coming back for over 15,000 years."

Layers just above the Buttermilk Creek discovery actually contained evidence of Clovis artifacts and occupations by other groups, and there was additional evidence of occupation continuing almost to historic times, the authors said.

"But the kicker was the discovery of nearly 16,000 artifacts below the Clovis horizon that dated to 15,500 years ago," Waters said.

Since the early 20th century, archaeologists have argued that the big-game hunting Clovis people – makers of distinctive and beautiful spear points that scientists can readily identify on sight — were the first to colonize North America in an overland migration from Asia about 13,000 years ago. But in recent decades, other evidence has eroded support for the so-called "Clovis first" theory. Sites in Wisconsin, Pennsylvania, Florida, Oregon and Monte Verde, Chili, all contain artifacts that some scientists believe pre-date Clovis, although some of these interpretations remain controversial.

The Buttermilk Creek discovery may help settle the issue once and for all.

"This early occupation of North America provides ample time for people to settle into the environments of North America, colonize South America by at least (14,100 to 14,600 years ago at Monte Verde, Chile), develop the Clovis tool kit, and create a base population through which Clovis technology could spread," the authors wrote.

Likewise, the school-book notion of the first Americans as intrepid mammoth hunters on the march through a continent-wide steppe that spanned south across Canada has also been challenged recently. Many archaeologists now argue that multiple migrations by many groups occurred over thousands of years — and these must have included coastal travelers.

"We're really interested and excited about this because of the fact that we're finally being able to put 'Clovis first' behind this and move on," Waters said

"We can start thinking about the routes that these people took into the Americas," he told the teleconference. "This lends indirect credence to the idea that people came along the coast and entered the Americas, and if that's the case, then perhaps people came up the Columbia River perhaps because that would have been the first major entry. … And we should look for early sites there. "

A case supporting a coastal migration route into the New World along Alaska isn't directly made by the new findings, but Waters said it follows logically from the unprecedented antiquity of the dates and other archaeological and geological work performed elsewhere on the continent.

First, genetic studies have conclusively linked the earliest Americans to populations that lived about the same time in Northeastern Asia, what is now the Russian Far East. The extensive Ice Age glaciation had locked up so much water that ocean levels were much lower, creating a 1,000-mile-wide isthmus between Alaska and Asia, the so-called Bering Land Bridge. Numerous archaeological sites in Alaska show that people with Asian connections ultimately moved into the state's Beringian steppes to hunt bison, mammoths and other game. But travel further east and south through Canada wouldn't have been possible until the continental ice sheets melted back and opened a corridor through central Canada about 12,000 to 13,000 years ago.

"While the Friedkin site does not directly tell us about migration routes taken by the first Americans, it does provide some information," Waters told Alaska Dispatch. "At 15,500 years ago and before, the Ice Free corridor between the two large ice sheets was closed. The glaciers had merged."

With ice sheets blocking travel between Asia and central North America through what is now Canada, the only available migration route would have run along Alaska's southern rim and then down the West Coast, Waters said.

"Hopefully someday an early site will be found close to the coast that demonstrates this," he said.